1. Field of the Invention
The present invention relates to a composite substrate containing a dielectric and an electrode, an electroluminescent (EL) device using the substrate, and a production process for the device.
2. Discussion of the Background
Electroluminescence is a phenomenon whereby a material emits light with the application of an electric field. Such a material is called electroluminescent (EL), and devices wherein this phenomenon is utilized have been put to practical use in liquid crystal displays (LCD) and back lights of watches.
EL devices are classified into two categories of a dispersion-type device and a thin-film device. The former has a structure in which a fluorescent material powder is dispersed in an organic material or an enamel and electrodes are disposed at top and bottom portions, and the latter contains a thin-film fluorescent material sandwiched between two electrodes and two thin-film insulators on an electrical insulating substrate. In addition, according to the type of driving system, each of the above two types of device is further classified into a direct-voltage drive system and an alternating-voltage drive system. The dispersion-type EL device has been known for a long time, and it has the advantage of facile preparation. However, the dispersion-type EL device has a low brightness and a short life, so that its utilization is limited. On the other hand, the thin-film EL device has a high brightness and a long life, and has, consequently, greatly expanded the practical application range of the EL device.
Heretofore, in the main type of thin-film EL device, a blue glass sheet for use in an LCD or a PDP is used as a substrate, transparent electrodes, such as ITO, are used as electrodes contacting the substrate, and light emitted from a fluorescent material is taken out from the side of the substrate. As the fluorescent material, Mn-containing ZnS capable of emitting yellowish orange light has been mainly used because it facilitates the formation of a film and has good light-emission properties. In order to fabricate a color display, it is essential to use fluorescent materials capable of emitting the primary colors of light, i.e., red, green and blue. As such materials, Ce-containing SrS and Tm-containing ZnS are selected for blue emission, Sm-containing ZnS and Eu-containing CaS are selected for red emission, and Th-containing ZnS and Ce-containing CaS are selected for green light emission, and research on these materials continues. Unfortunately, since they are insufficient in brightness, luminous efficiency and color purity, they have not been put to practical use.
In an effort to address these problems, it is considered that a process of forming a film at high temperature or heat treatment of a formed film at a high temperature is promising. However, when such a technique is used, it is impossible to use a blue glass plate as the substrate from the viewpoint of heat resistance. Although the use of a quartz substrate having heat resistance has also been investigated, the quartz substrate is very expensive, and, therefore, it is not suitable for an application, such as a display which requires a large area.
As disclosed in Japanese Patent Application Laid Open No. 50197/1995 and Japanese Patent Publication No. 44072/1995, there has recently been reported the development of a device in which a ceramic substrate having electrical insulation properties is used as a substrate and a thick-film dielectric is substituted for the thin-film insulator located at the lower portion of a fluorescent material.
The basic structure of this device is shown in FIG. 8. The EL device shown in FIG. 8 has a structure in which a lower electrode 12, a thick-film dielectric layer 13, a luminescent layer 14, a thin-film insulator layer 15 and an upper electrode 16 are successively formed on a substrate 11 made of ceramic or the like. Accordingly, in contrast to the structure of the conventional thin-film EL device, a transparent electrode is placed at the top portion in order to take out light of the fluorescent material from the top portion opposite to the substrate.
The thick-film dielectric layer of this device has a thickness of several tens micrometers, which is several hundreds to several thousands times as much as that of the thin-film insulator layer. Therefore, the breakage of the insulator caused by pinholes or the like can be inhibited. Thus, the above device has the advantage that a high reliability and a high yield at the time of production can be obtained.
Voltage drop across the luminescent layer caused by the use of a thick dielectric can be prevented by forming the dielectric layer from a material having a high dielectric constant. Further, the rise of a heat treatment temperature can be allowed by using the ceramic substrate and the thick-film dielectric. As a result, the film formation of a highly luminous material, which has heretofore been impossible owing to the presence of defective crystals, is made possible.
However, when the substrate, the electrode and the dielectric layer are to be laminated by a thick-film forming process, the surface of the dielectric layer becomes inconveniently uneven in some cases.
In the conventional process, a substrate/electrode/dielectric layer composite substrate is obtained by first forming the electrode on the substrate of alumina or the like in a predetermined pattern by a thick-film forming process such as a print process, forming the dielectric layer on the electrode by the thick-film forming process, and then sintering the whole laminate obtained.
However, as shown in FIG. 9, for example, there has been a concern that the surface of the dielectric layer 13 may be uneven owing to the differences in shrinkage ratios and thermal expansion coefficients between the electrode layer 12 and the dielectric layer 13 when the electrode layer 12 is formed in a predetermined pattern. Furthermore, the surface of the dielectric layer 13 is cracked in some cases owing to the difference in thermal expansion coefficients between the substrate 11 and the dielectric layer 13. Thus, when the dielectric layer 13 has an uneven or cracked surface, the thickness of the dielectric layer 13 becomes non-uniform, or a peeling phenomenon occurs between the dielectric layer 13 and the luminescent layer formed thereon, whereby the performance and the display quality of the device are remarkably impaired.
Therefore, in the conventional process, it is necessary to remove large uneven portions by grinding, for example, and fine uneven portions by a sol-gel process.
Accordingly, it is an object of the present invention is to provide a composite substrate in which the an insulating layer does not become uneven by the influence an electrode layer and which requires neither a grinding process nor a sol-gel process, is easy to produce, and can provide a thin-film EL device having a high display quality when applied thereto.
It is another object of the present invention is to provide a thin-film EL device using the above substrate.
It is yet another object of the present invention is to provide a production process for the above device.
The above objects of the present invention are achieved by the following composite substrates, devices and processes.
(1) A composite substrate containing a substrate; an electrode layer embedded in the substrate in such a manner that the electrode layer and the substrate are in one plane: and an insulating layer formed on the surface of a composite of the substrate and the electrode layer.
(2) A composite substrate according to the above (1), wherein the insulating layer contains a dielectric having a dielectric constant of 1000 or more.
(3) A composite substrate according to the above (1) or (2), wherein the insulating layer contains barium titanate as a main component.
(4) A composite substrate according to the above (3), wherein the insulating layer contains, as a secondary component, at least one selected from the group consisting of magnesium oxide, manganese oxide, tungsten oxide, calcium oxide, zirconium oxide, niobium oxide, cobalt oxide, yttrium oxide and barium oxide.
(5) A composite substrate according to the above (3) or (4), wherein the insulating layer contains, as a secondary component, at least one selected from the group consisting of SiO2, MO, and B2O3, wherein M is at least one element of Mg, Ca, Sr and/or Ba.
(6) A composite substrate according to any one of the above (1) to (5), wherein the insulating layer contains barium titanate as a main component and at least one selected from the group consisting of magnesium oxide, manganese oxide, yttrium oxide, barium oxide and calcium oxide, and silicon oxide as secondary components; and the content of magnesium oxide in terms of MgO is 0.1 to 3 moles, that of manganese oxide in terms of MnO is 0.05 to 1.0 mole, that of yttrium oxide in terms of Y2O3 is not more than 1 mole, that of barium oxide in terms of BaO and calcium oxide in terms of CaO is 2 to 12 moles, and that of silicon oxide in terms of SiO2 is 2 to 12 moles, based on 100 moles of barium titanate in terms of BaTiO3.
(7) A composite substrate according to the above (3), wherein the total content of BaO, CaO and SiO2 in terms of (BaxCa1-xO)y.SiO2 (provided that x satisfies 0.3xe2x89xa6xxe2x89xa60.7 and y satisfies 0.95xe2x89xa6yxe2x89xa61.05) is 1 to 10 wt % based on the total content of BaTiO3, MgO, MnO and Y2O3.
(8) A composite substrate according to any one of the above (1) to (7), which is a thick film obtained by sintering the laminate formed by the use of a sheet-forming process or a print process.
(9) A composite substrate according to any one of the above (1) to (8), which is obtained by forming a functional film on the insulating layer, and then heating the functional film at a temperature of from 600xc2x0 C. to a sintering temperature of the substrate or less.
(10) A thin film EL device comprising the composite substrate in any one of the above (1) to (6), and a luminescent layer, another insulating layer and another electrode layer formed successively on the composite substrate.
(11) A thin film EL device according to the above (10), wherein the electrode layer contains at least one element of Ag, Au, Pd, Pt, Cu, Ni, W, Mo, Fe and/or Co; or at least one alloy of Agxe2x80x94Pd, Nixe2x80x94Mn, Nixe2x80x94Cr, Nixe2x80x94Co and/or Nixe2x80x94Al alloys.
(12) A process for producing a thin film EL device entailing:
forming a first insulating layer precursor on a film sheet having a flat surface by a thick-film production process;
forming a first patterned electrode layer precursor thereon;
forming a substrate precursor thereon, subjecting the laminate to a binder-removing treatment and sintering it to obtain a composite substrate having the first electrode layer and the first insulating layer formed on the substrate; and
further laminating a luminescent layer, a second insulating layer and a second electrode layer on the first insulating layer successively to obtain the thin-film EL device.
(13) A process for producing the thin film EL device according to the above (10), wherein a heat treatment is carried out at a temperature of from 600xc2x0 C. to a sintering temperature of the substrate or less, after the formation of the second insulating layer or the second electrode layer.
(14) A process for producing the thin film EL device according to the above (12) or (13), wherein the substrate precursor is a substrate green sheet which contains at least one selected from the group consisting of alumina (Al2O3), silica glass (SiO2), magnesia (MgO), steatite (MgO. SiO2), forsterite (2MgO, SiO2), mullite (3Al2O3.2SiO2), beryllia (BeO), zircon, and Ba-, Sr- and Pb-based perovskites.
(15) A process for producing the thin film EL device according to any one of the above (12) to (14), wherein the composition of the main component of the substrate precursor is the same as that of the insulating layer.
(16) A process for producing the thin film EL device according to any one of the above (12) to (15), wherein the electrode layer precursor comprises at least one selected from the group consisting of Ag, Au, Pd, Pt, Cu, Ni, W, Mo, Fe and Co, or any one of Agxe2x80x94Pd, Nixe2x80x94Mn, Nixe2x80x94Cr, Nixe2x80x94Co and Nixe2x80x94Al alloys.
(17) A process for producing the thin film EL device according to any one of the above (12) to (16), wherein the sintering temperature is in a range of 1,100 to 1,400xc2x0 C.